The present invention pertains to aqueous based adhesives having improved shear strength.
Due to rising costs and environmental concern there has been significant interest in developing adhesives which do not utilize hydrocarbon solvents. There are two options available to adhesive manufacturers who wish to minimize the use of hydrocarbon solvents. The manufacturer may use a hot melt adhesive or an aqueous based adhesive. Aqueous based adhesives are relatively simple to use and do not require extensive equipment modification over solvent based adhesive systems. Aqueous based adhesives have come under the criticism that they do not provide sufficient shear strength. There exists a need to develop aqueous based adhesives, and particularly pressure sensitive adhesives having improved shear strength.
It has been found that the shear strength of adhesives based on latexes containing functional groups such as carboxylic acid, amide, aldehyde and anhydrides of dicarboxylic acids, may be improved by the post polymerization neutralization of such latexes with a monovalent metal alkali. Without wishing to be bound by theory, it is believed that the size of the monovalent ion contributes to the shear strength developed and that the smaller the monovalent metal ion the higher the shear strength.
There is a significant body of art relating to the use of multi-valent metal ions, amines or diamines to cross link polymers. Such methods are disclosed in Polysar Limited's Canadian Pat. Nos. 1,017,492 and 1,030,699 issued Sept. 13, 1977 and May 2, 1978, respectively, relating to high green strength rubber. In the latex field the use of multi-valent ions, amines and diamines to coagulate and/or cross link latex has also been disclosed.
U.S. Pat. No. 2,754,280 issued July 10, 1956 assigned to the Rohm and Haas Company discloses the formation of crosslinked films by coagulated dispersions of polymers containing acid or ester functional groups with a polyvalent metal ion. The coagulated films are "to all intents and purposes dispersions of crosslinked resins because pairs of carboxyl groups in various molecular chains are joined together, i.e. are crosslinked by the atoms of the polyvalent metals "(col. 1 lines 57 to 61)". A similar process is disclosed in U.S. Pat. No. 2,959,821 issued Nov. 15, 1960 and assigned to Farbenfabriken Bayer Aktiengesellschaft.
Processes for preparing latex useful as a binder in carpet backing applications are known in which a carboxylated latex is neutralized with ammonium hydroxide. Such latices are cured in the absence of heavy metal ions. These types of processes are disclosed in Canadian Pat. No. 808,713 issued Mar. 18, 1969 to the Dunlop Rubber Company and East German Pat. No. 137,593 published July 10, 1978 in the name of VEB Chemische Werke Buna.
It is also known to impregnate webs with a latex of a functional polymer and to expose the impregnated web to amine vapour. The amine produces an in situ "thickening" of the latex.
The above art all teaches away from the use of monovalent ions as crosslinking agents. The conventional theory of ionic crosslinking requires a divalent metal compound or an ammonium complex. Thus, the use of a monovalent ion to cross link a functional polymer goes against the current state of the art.
British Patent Specification No. 1,085,833 published Oct. 4, 1967 in the name of Uniroyal discloses an adhesive for corrugated paper board comprising a carboxylated latex, a proteinaceous or carbohydrate binder and a crosslinking agent. The preferred crosslinking agents are urea formaldehyde or melamine formaldehyde resins. Complexes of polyvalent metal ions are also suitable as crosslinking agents. This art teaches away from the present case in that it does not suggest the use of a monovalent metal ion as a crosslinking agent. Furthermore there is no teaching that the carboxylated latex should be neutralized with a fixed alkali.
U.S. Pat. No. 3,677,991 issued July 18, 1972, assigned to the Dow Chemical Company discloses a method of gelling a latex high in acrylate and containing from about 1.5 to 8 percent of a carboxylic acid monomer. The latex is irreversibly coagulated with a strong base, preferably ammonium hydroxide, but sodium hydroxide, potassium hydroxide and ethylene diamine are also suitable. The patent teaches that surfactant coverage on the latex particles is less than 100%. Coagulation does not take place until the pH is from 8.8 to 10.1. The patent does not teach that the gelled latex is useful as an adhesive.
Japanese Kokai No. 76/38,321 published Mar. 31, 1976 in the name of Dainippon Ink and Chemicals discloses a latex of a polymer containing from 20 to 80% of unsaturated carboxylic acid, 5 to 50% of butadiene and 0 to 75% of other comonomers. The latex when neutralized with ammonium hydroxide to a pH of 9 is useful as an adhesive for tiles.
Japanese Kokai No. 83/31,972 published Aug. 5, 1980 in the name of Sango-Kokusaku Pulp Co. Ltd. discloses a composition for use as a release coating. The composition is a latex of a copolymer of a C.sub.4-10 alkylacrylate, and ethylenically unsaturated carboxylic acid, and a vinyl compound (ethylene dimethacrylate) which has been treated with 0.2 to 1 equivalents of sodium acetate per equivalent of carboxylic acid. The document teaches that sodium ions provide better release properties than those obtained with calcium ions. The polymer of the Japanese Kokai contains ethylene dimethacrylate, a crosslinking agent, rather than the C.sub.2-4 mono-olefin as required in the present invention.
British Pat. No. 848,808, published Sept. 21, 1960 in the name of the International Latex Corporation, teaches a binder for non-woven webs. The binder comprises 60 to 75% of one or more dienes of from 4 to 10 atoms, from 1 to 30% of a vinyl compound and from 1 to 39% of a monocarboxylic acid. Following polymerization the latex may be neutralized with ammonium hydroxide, an organic base or sodium hydroxide. The patent does not disclose any preference for neutralization with sodium hydroxide. Furthermore, the patent does not teach that such latices are useful as adhesives.